1. Field
The present disclosure relates to methods and apparatus for estimating a sleep clock frequency and, more particularly, estimating a sleep clock frequency using a high performance clock such as a temperature compensated crystal oscillator (TCXO).
2. Background
Mobile transceivers, such as mobile phones, typically employ a temperature compensated oscillator (TCXO) that provides very accurate timing for various functions within the device including keeping the system time. Clocks such as a TCXO, however, use a relatively large amount of power, drawing approximately 1.5 mA of current. In order to improve the battery life of a mobile transceiver, it is known to place most current consuming units within the device into a power saving mode and maintain the system time using low-power sleep circuits. Because of the high current draw of a TCXO, it is not energy efficient to use such a device to maintain system time for sleep circuits.
Accordingly, it is known to maintain system timing during sleep or power saving modes by using a sleep controller having a much lower power usage (e.g., a clock with a current draw of 200 μA) and a lower frequency (e.g., 30-60 kHz) than TCXO devices. This is typically accomplished with a cost effective crystal oscillator clock at the expense of some accuracy in time keeping because the clock frequency tends to fluctuate. This clock is otherwise known as the “sleep clock” or “slow clock.”
When a transceiver wakes up from a sleep mode, it is important to have an accurate system time as kept by the sleep clock. Since the slow clock is used for system timing during sleep modes, the accuracy of the clock timing will directly affect the system time when the mobile transceiver wakes up prior to re-acquisition of timing based on information received from the wireless network, such as a CDMA based network. A good estimate of slow clock frequency is therefore desirable. Known timing estimation utilized by mobile devices, however, is typically used only for initial calibration and the slow clock time tracking is solely dependent on Pseudo Noise (PN) code timing. In certain wireless systems not employing PN timing (e.g., Orthogonal Frequency Division Multiplexing (OFDM)), however, this timing in not available. Thus, in such systems the accuracy of the sleep clock timing is even more important. In the case of OFDM, in particular, such systems are more susceptible to timing errors such as synchronization timing made worse by intersymbol interference.